Developing electrostatic images employing fatty acid esters to inhibit developer build-up

ABSTRACT

AN ELECTROSTATOGRAPIC IMAGING SYSTEM WHEREIN A HYDROGENEATED VEGETABLE OIL DERIVATIVE IS ADDED TO THE PHOTOCONDUCTOR SURFACE IN THE PRESENCE OF A LIQUID DEVELOPER. THE ADDITIVE MAY BE SEPARATELY ADDED OR BE DISPERSED IN THE LIQUID DEVELOPER AND TO PROVIDE IMPROVED CYCLING ABILITY OF THE IMAGING SURFACE.

United States Patent thee 3,692,520 Patented Sept. 19, 1972 3,692,520 DEVELOPING ELECTROSTATIC IMAGES EMPLOY- ING FATTY ACID ESTERS T INHIBIT DE- VELOIER BUILD-UP Joseph Mammino and Alan B. Amidon, Penfield, N.Y., assignors to Xerox Corporation, Rochester, N.Y. No Drawing. Filed Oct. 31, 1969, Ser. No. 873,105 Int. Cl. 1305c 1/16, 1/20; G03g 9/04 U.S. CI. 96-13 19 Claims ABSTRACT or THE DISCLOSURE An electrostatographic imaging system wherein a hydrogenated vegetable oil derivative is added to the photoconductor surface in the presence of a liquid developer. The additive may be separately added or be dispersed in the liquid developer and to provide improved cycling ability of the imaging surface.

BACKGROUND OF THE INVENTION This invention relates to imaging systems, and more particularly, to improved developer systems and techniques.

The formation and development of images on the sur-. face of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely-divided electroscopic material referred to in the art as toner. The toner will normally be attracted o those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently afiixed to a support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.

Similar methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. Included within this group are the cascade" development technique disclosed by E. N. Wise in U.S. Pat. 2,618,552; the powder cloud technique disclosed by C. F. Carlson in U.S. Pat. 2,221,776 and the magnetic brush process disclosed for example in U.S. Pat. 2,874,063.

Development of an electrostatic latent image may also be achieved with liquid rather than dry developer materials. In conventional liquid development, more commonly referred to as electrophoretic development, an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with the charged image pattern the suspended particles migrate toward the charged portions of the imaging surface separating out of the insulating liquid. This electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration.

A significant advance in developing electrostatic latent images in a liquid development process is disclosed by R. W. Gundlach in U.S. Pat. 3,084,043. In this method, hereinafter referred to as polar liquid development, an electrostatic latent image is developed or made visible by presenting to the imaging surface a liquid developer on the surface of a developer dispensing member having a plurality of raised portions or lands defining a substantially regular patterned surface and a plurality of portions depressed below the raised portions or valleys. The depressed portions of the developer dispensing member contain a layer of conductive liquid developer which is maintained out of contact with the electrostatographic imaging surface. Development is achieved by moving the developer dispensing member loaded with liquid developer in the depressed portions into developing configuration with the imaging surface. The liquid developer is believed to be attracted from the depressed portions of the applicator surface in the charged or image areas only. The developer liquid may be pigmented or dyed. The development system disclosed in U.S. Pat. 3,084,043, differs from electrophoretic development systems where a substantial contact between the liquid developer and both the charged and uncharged areas of an electrostatic latent image bearing surface occurs. Unlike electrophoretic development systems, substantial contact between the polar liquid and the areas of the electrostatic latent image bearing surface not to be developed is prevented in the polar liquid development technique. Reduced contact between a liquid developer and the non-image areas of the surface to be developed is desirable because the formation of background deposits is thereby inhibited. Another characteristic which distinguishes the polar liquid development is the fact that the liquid phase of a polar developer ment technique from electrophoretic development is the fact that the liquid phase of a polar developer actually takes part in the development of a surface. The liquid phase in electrophoretic developers functions only as a carrier medium for developer particles.

An additional development technique is that referred to as wetting development described in U.S. Pat. 3,285,741. In this technique an aqueous developer uniformly contacts the entire imaging surface and due to the selected wetting and electrical properties of the developer substantially only the charged areas of the imaging surface are wetted by the developer. The developer should be relatively conductive having a resistivity generally from about 10 to 10 ohm-cm. and have wetting properties such that the wetting angle measured when placed on the photoconductor surface is smaller than at the charged areas and greater than 90 in the uncharged areas.

While capable of producing satisfactory images the liquid development systems in general suffer deficiencies in certain areas and are in need of further development and improvement. Particularly troublesome difficulties are encountered in liquid development systems employing a reusable or cycling photoconductor surface. In these systems a photoconductor surface such as a selenium or selenium alloy drum is charged, exposed to a light and shadow image and developed by bringing the image bearing surface into developing configuration with an applicator containing developing quantities of liquid developer thereon. The liquid developer is transferred according to the appropriate technique from the developer applicator onto the image bearing surface to develop the image. Thereafter, the developer on the photoconductor surface is transferred in image configuration to copy paper where the liquid developer may be absorbed by the paper to form a permanent print. During the transfer operation all the liquid developer is not transferred to the copy paper and a considerable quantity remains on the photoconductor surface. The photoconductor drum on further movement may come in contact with a cleaning device which may be in the form of an absorbent web material. This absorbent web smoothes out the liquid developer on the photoconductor surface to the configuration of a thin layer while absorbing excessive quantities. The photoconductor is then in position to commence the charge, expose, develop, transfer and clean sequence once again. On repeated cycling there is a progressive accumulation of liquid developer on the photoconductor surface since in each cycle all the developer is not transferred to the copy paper. This progressive accumulation of old developer results in an overall loss of density, deterioration of fine detail and contributes to increased background deposits on the final copy particularly since accurate imaging on the photoconductor is inhibited. This accumulation may be viewed as a buildup of layers for each cycle some of which may transfer to the copy paper progressively yielding poorer quality prints and frequently producing no image at all after only a few cycles.

*Procedures to remove the developer liquid from the surface of the photoconductor have been employed. However, to provide the necessary removal of ink film the cleaning step must be so severe and complete that there is a progressive degradation of the photoconductor surface lessening its useful life span. The severity of the cleaning step is dictated by the fact that in cleaning a film from a surface the film is progressively split such that on each separate cleaning about one half the film remains on the photoconductor surface. In some instances and with complete removal of the ink film the electrical properties of the photoconductor are virtually destroyed by the cleaning operation after only a small number of cycles.

Particularly troublesome is the fact that with progressive cycling and cleaning the electrical properties of the photoconductor are progressively destroyed. Particularly noticeable is the fact that the photoconductor holds less and less charge resulting in progressively deteriorating resolution of the developed image and transferred copy prints. These disadvantages are particularly noticeable when employing a selenium or selenium alloy photoconductor. With complete cleaning of the imaging surface after the first cycle, which produces a print of good quality, the print obtained in the second cycle is of markedly poorer quality and by the time the print from the third or fourth cycle is obtained little image definition may be observed to remain. Copending application Ser. No. 838,328 of J. Mammino and A. Amidon filed July 1, 1969, now abandoned, entitled Imaging Systems, describes a technique wherein the electrical properties of a cycling photoconductor may be cyclicly rejuvenated by the addition of a Lewis acid or base to the liquid developer or to the photoconductor surface. This technique, however, still requires an extensive and virtually complete cleaning of the photoconductor in every imaging cycle to obtain prints of consistent quality on every cycle.

Additional areas of difliculty persist in liquid development systems employing both cycling or reusable photoconductor or single use photoconductor. Difficulty is frequently experienced in obtaining and maintaining final copies of consistent print quality. One factor aflfecting print quality is that which is referred to as feathering or lateral flow of the developer where the liquid developer tends to slowly spread on the surface of the copy paper resulting in a less sharp image and decreased resolution.

Difficulty is also experienced with the ink stability with regard to settling out of the pigment on aging and evaporation of the developer vehicle. These difficulties are particularly significant in a copier employing a liquid development technique where the copier is subjected to long term intermittent use.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a developing system which overcomes the above noted deficiencies.

It is another object of this invention to provide a liquid development system which produces cleaner, sharper first copies of reduced background deposits.

It is another object of this invention to provide a liquid development system in which a reusable photoconductor does not have to be completely cleaned after each cycle.

It is another object of this invention to provide a liquid development system having reduced Wear on the photoconductor surface.

It is another object of this invention to provide a liquid development system having reduced lateral spreading of developer on the copy paper.

It is another object of this invention to provide liquid developers having improved cycling ability.

It is another object of this invention to provide a liquid developer capable of producing copies of improved print quality.

It is another object of this invention to provide a stable liquid developer.

It is another object of this invention to provide an inexpensive and operationally efficient liquid development system employing a cycling photoconductor.

It is another object of this invention to provide a liquid developer having improved flow characteristics.

It is another object of this invention to provide a liquid development system superior to known systems.

It is another object of this invention to provide liquid developers superior to known developers.

The above objects and others are accomplished, generally speaking, by providing an electrostatographic imaging system wherein a hydrogenated vegetable oil derivative is added to the photoconductor surface in the presence of a liquid developer. More specifically, in a liquid development system employing a reusable photoconductor the hydrogenated vegetable oil derivative may be added to the developer film on the cycling photoconductor surface or it may be added as a constituent of the liquid developer dispersed or suspended in the vehicle. In other liquid development systems the additive may be added as a constituent of the liquid developer.

The hydrogenated vegetable oil derivatives employed in the practice of this invention may generally be described as fatty acid esterified alkylene diols and triols. While the alkylene diols and triols need not be completely esterified, it is preferred in obtaining the desired properties to employ them in at least a partially esterified form and preferable in the form of at least about the mono ester.

Any suitable esterified alkylene diol or triol may be employed. Typical materials generally have the following structure:

with x being 0 to 8 preferably 0 to 4 and y being 0 to 7 and preferably 0 to 3; R R and R, are hydrogen or the group and R is the carbon chain of a saturated or unsaturated fatty acid of from about 12 to about 22, preferably 17 to 19, carbon atoms that may be hydroxy substituted. In any single compound it is provided that at least one of R R or R includes the group The above preferred limitations or constituent chain length generally define those materials possessing particularly effective flow properties.

Typical materials within this group include among others the mono, di, and tri stearates, oleates, laurates, palmitates, linoleates, linolenates, ricinoleates and mixtures thereof of the alkylene diols and triols of from 2 to carbon atoms. Typical diols and triols that may be at least partially esterified include among others ethylene glycol diricinoleate, ethylene glycol monoglycol, 2,3 butylene glycol, 1,2,4 butanetriol, 1,5- pentanediol, 1,6 hexanediol, 1,2,6 hexanetriol, 1,7- heptanediol, 1,8 octanediol, 1,9 nonanediol, 1,10- decanediol.

Specific typical materials include ethylene glycol monostearate, ethylene glycol mono-(hydroxy stearate), ethylene glycol distearate, ethylene glycol monoalaurate, ethylene glycol dilaurate, ethylene glycol monooleate, ethylene glycol dioleate, ethylene glycol monoricinoleate, ethylene glycol direcinoleate, ethylene glycol monopalmitate, ethylene glycol dipalmitate, propylene glycol mono-(hydroxy stearate), propylene glycol monostearate, propylene glycol distearate, glyceryl tri-(l2-hydroxy stearate), glyceryl tristearate, glyceryl monostearate, glyceryl 1,3-distearate, glyceryl triricinoleate, glyceryl monooleate, glyceryl monoricinoleate, glyceryl monopalmitate, glyceryl 1,3 dipalmitate, glyceryl tripalmitate, glyceryl monolaurate, glyceryl monolinoleate, glyceryl dilinoleate, glyceryl trilinolenate, glyceryl monolinoleate, glyceryl dilinoleate, glyceryl trilinoleate, 1,4 butylene glycol monooleate, 1,4 butylene glycol dipalmitate, 1,4- butylene glycol distearate, 1,4 butylene glycol mono- (hydroxy stearate), 1,4 butylene glycol monoricinoleate, 2,3 butylene glycol dilaurate, 2,3 butylene glycol diricinoleate, 2,3 butylene glycol monopalmitate, 1,2,4- butanetriol tristearate, 1,2,4 butanetriol 1,4 dilaurate, 1,2,4 butanetriol 2 monooleate, 1,2,4 butanetriol- 2,4 diricinoleate, 1,5 pentanediol dilinoleate, 1,5- pentanediol mono-(hydroxy stearate), 1,5 pentanediol dipalmitate, 1,5 pentanediol dioleate, 1,6 hexanediol diricinoleate, 1,6 hexanediol monolaurate, 1,2,6 hexanetriol tristearate, 1,2,6 hexanetriol 1,6 dioleate, 1,8 octanediol diricinoleate, 1,8 octanediol monostearate, 1,8 octanediol monolaurate, 1,8 octanediol monolinoleate, 1,9 nonanediol distearate, 1,9-nonanediol monooleate, 1,10 decanediol dilinoleate, and 1,10-

Materials producing particularly satisfactory cycling ability and print quality and therefore preferred may generally be described as lower alkyl esters of hydrogenated hydroxyoctadecenoic acid and may include the mono, di and tri alkyl esters and mixtures thereof. The term lower alkyl includes straight and branched chains of from 1 to about 10 carbon atoms, preferably from about 2 to about 6 carbon atoms. Within this group, materials having particularly effective flow properties include glyceryl tri (12-hydroxy stearate), propylene glycol monohydroxy stearate and ethylene glycol monohydroxy stearate.

In a liquid development system employing a recycling or reusable electrostatographic imaging surface, the developer on the imaging surface is transferred to a receiving surface for each cycle. During the transfer operation, all the liquid developer is not transferred to the copy paper and a considerable quantity remains on the imaging surface. Following transfer, the photo-conductor may be cleaned with any suitable cleaning technique which on contact with the photoconductor smoothes out the liquid developer on the photoconductor surface to the configuration of a thin layer while absorbing excessive quantities. The photoconductor is then in position to commence the charge, expose, develop, transfer and clean sequence necessary for repeated cycling. On repeated cycling the quantity of liquid developer builds up on the photoconductor until virtually all image, fine detail and density is lost and background deposits are great. This frequently will occur within the first three or four cycles. The resultant image transferred to copy paper is of equal quality as will be readily appreciated since large areas of the photoconductor surface other than the image defining area retain liquid developer some of which will transfer to the copy paper. With each cycle an additional layer of developer is produced on the photoconductor surface. It is generally observed that residual layers of liquid developer more than about one micron thickness will transfer to the copy paper and since this condition is rapidly reached in a cycling system the resultant quality of a continuous series of prints is poor.

However, with the addition of a small amount of a fatty acid esterified alkylene diol or triol to the liquid developer a steady state or static situation is obtained. With the presence of a small quantity of the additives of this invention residual layers of liquid developer of up to about three microns are formed on the photoconductor before the developer will transfer to the copy paper. However, at this thickness no continued buildup of liquid developer occurs on the photoconductor surface since a simple cleaning web will remove the excess over this thickened layer of about three microns.

The mechanism by which the result is achieved is not fully understood. The additives of this invention however are believed to impart non-newtonian flow properties known as pseudoplasticity or thixotropy to the developer liquid. In exhibiting these flow characteristics the developer liquid becomes more fluid at times and during those operations of the imaging cycle where increased fluidity is desired while becoming more viscous significantly at other appropriate times and operations in the cycle where it is desired to maintain the liquid pattern localized or relatively immobile. When subjected to shearing forces a developer liquid containing the esters of this invention will become more fluid and will spread with relative ease on a surface. When not subjected to shearing forces the same developer has high resistance to flow and is relatively fixed or rigid on a surface.

In operation, the liquid developer of this invention containing a dispersed powdered fatty acid esterified alkylene diol or triol is applied to a developer applicator surface. The applicator surface is doctored with any suitable means to provide a uniform supply of liquid developer on the applicator surface. Preferably, the liquid developer is uniformly present in the depressed portions or valleys of an applicator surface While the raised portions or lands are substantially free of developer to thereby minimize areas of background deposit and insure adequate supply of liquid developer to the entire image bearing surface. During the doctoring operation the doctoring surface is placed against the applicator surface such that the liquid developer is subjected to a shearing force and due to the pseudo-plastic or thixotropic nature of the liquid developer the liquid becomes more fluid and will be more readily uniformly distributed on the applicator surface according to the above preferred configuration. The developer loaded applicator surface may then be placed in developing configuration with the image hearing surface such that the liquid developer is pulled out of the depressed portions of the applicator surface in the charged or image areas only. This developing configuration may include contact or no contact between the photoconductor and the applicator surface, the important factor being only that developing quantities of the developer be attracted to the charged or image areas of the photoconductor. The developer on the photoconductor in image configuration is then transferred to copy paper. During this step all the developer is not transferred to the copy paper. The photoconductor with residual developer present on its surface is then brought into contact With an absorbent cleaning Web. 'During this cleaning procedure the shearing forces placed on the developer liquid are sufficient to make the developer liquid more fluid thereby enabling spreading of the developer uniformly over the photoconductor surface. Thereafter the cycle of charging, exposing, developing, transferring and cleaning is repeated. On repeating cycling the film of liquid developer containing a small amount of the hydrogenated vegetable oil derivatives increases layer by layer for each cycle up to a maximum of about three microns. Thereafter on repeating cycling no further buildup of layers is obtained as a steady state condition is reached. When this point is reached, in each cycle liquid developer is applied to the photoconductor surface in image configuration; transferred to the copy paper in image configuration and thereafter cleaned by a cleaning web. During the cleaning operation the residual developer from the newly applied developer in image configuration is smoothed out and removed by the cleaning web resulting in a photoconductor surface having substantially the same amount of liquid developer on its surface as before the last development step.

The steady state or static condition is reached rapidly after only a few cycles and thereafter substantially no further accumulation of liquid developer on the photoconductor surface is achieved. During this process the liquid developer containing the additives of this invention builds up layer by layer to a maximum of about three microns such that there is present a plurality of layers of gel-like structure with very little mixing between layers. When the static condition is reached substantially only the newest layer of liquid developer, that which is attracted to the photoconductor in image configuration from the applicator surface, is transferred to the copy paper. Any residual developer from this layer may be removed with the simple cleaning web. When this static condition is achieved the shearing forces present during cleaning are sufficient to impart the necessary fluidity to achieve the removal of residual developer from the photoconductor surface during any complete cycle.

By the term liquid developer it is intended to refer to the entire developer composition which may comprise one or more vehicles, one or more pigments, dispersants and other materials in addition to the hydrogenated vegetable oil derivatives of this invention.

Any suitable vehicle may be employed including polar and non-polar liquids, aqueous and nonaqueous systems. In general, developers having conductivities of from about to about 10- (ohm-cm.) may be employed. Typical materials that may be employed as principal vehicles include glycerol, water, 2,5-hexanediol, poly propylene glycol, mineral spirits, 2-ethyl-l,3-hexanediol, mineral oil, benzyl alcohol, dipropylene glycol, parafiin oil, oleic acid, dihexyl phthalate, vegetable oils such as castor oil, rapeseed oil, sesame oil, cottonseed oil, corn oil, sunflower seed oil, olive oil, and peanut oil. Also included are fluorocarbon oils such as Du Ponts Freon solvents and Krytox oils, silicone oils, kerosene, carbon tetrachloride, toluene.

In development systems employing a reusable or cycling photoconductor it is preferred to employ a relatively insulating developer in order to provide adequate charge retention on the photoconductor. If, for example, a highly conductive developer is employed it will discharge a charged photoconductor when placed on the photoconductor. Further, since a thin film of developer is present on the surface of a recycling photoconductor adequate charge retention of the photoconductor for each cycle is difiicult to maintain. For these reasons, it is preferred to employ a vehicle having a conductivity less than about 10* (ohm-cm.)- Any hydrocarbon oil having this hydrocarbon oil having this conductivity may be used. Typical vehicles within this group include mineral oil, the vegetable oils including castor oil, peanut oil, sunflower seed oil, corn oil, rapeseed oil, and sesame oil. Also included are oleic acid, kerosene, silicone oils and fluorocarbon oils.

In addition to the above principal vehicles, an auxiliary or secondary vehicle may be employed to impart or adjust any one or more of the properties of the principal vehicle.

Any suitable material may be employed as the secondary vehicle and may, for example, have dispersant properties, contribute to viscosity adjustment, or confer wetting properties to the pigment employed. In addition, the secondary vehicles preferably exhibit properties in common with the principal vehicle of being nonodorous, non-hydroscopic, and of low volatility to provide a stable developer with a nonotfensive odor. An additional function of secondary vehicles may be to help thed eveloper penetrate into the photoconductor or copy paper. Typical materials that may be employed as either primary or secondary vehicles include dibutyl phthalate, butyl isodecyl phthalate, butyl octyl phthalate, diisooctyl phthalate, di-2-ethyl phthalate, isooctyl isodecyl phthalate, normal octyl phthalate, diisodecyl phthalate, ditridecyl phthalate, isodecyl tridecyl phthalate, diisooctyl adipate, di-Z-ethyl hexyl adipate, isooctyl isodecyl adipate, normal octyl decyl adipate, diisodecyl adipate, diisooctyl sebacate, di 2 ethyl hexyl sebacate, polyadipiate ester, isooctyl palmitate, butyl stearate, butyl oleate, triethylene glycol dicaprylate, triethylene glycol caprylatecaprate, triethylene glycol dipelargonate, diethylene glycol dipelargonate, butanediol dicaprylate, triisooctyl trimellitate, tri l-ethyl hexyl trimellitate, mixed normal trialkyl trimellitate.

Any suitable colorant may be employed in the developer including both pigments and dyes. It is preferred that the colorant be fast to light in order to obtain image permarence. Typical pigments include carbon black, charcoal and other forms of finely divided carbon, iron oxide, ultramarine blue, zinc oxide, titanium dioxide, and benzidine yellow. Typical dyes include methylene blue, methyl violet, oil red, oil blue and oil yellow.

A dispersant is generally employed to aid in dispersing the pigment and other additives in the vehicles. Any suitable dispersant may be employed that is compatible with and soluble in the vehicle. Typical materials include alkylated polyvinyl pyrrolidone and copolymers of n-octadecyl vinyl, ether and maleic anhydride.

When the developer is to be employed in a development system having a recycling or reusable photoconductor it is preferred to add a small amount of an appropriate Lewis acid or base according to the technique described by J. Mammino and A. Amidon in application Ser. No. 838,328 entitled Imaging Systems and filed July 1, 1969, now abandoned. Disclosed therein is a method of cyclicly rejuvenating the electrical properties of reusable photoconductor wherein small amounts of any suitable Lewis base are added to the liquid developer used in conjunction with positively charged photoconductors and small amounts of any suitable Lewis acid is added to the developer used with negatively charged photoconductors.

Typical examples of Lewis bases include among others the triarylmethane dyes such as crystal violet, malachite green, para-rosaniline, basic fuchsin; xanthene dyes such as rhodamine B, iosin, erythrosine and fluoroscein; aniline dyes such as nigrosine and aniline black, soluble porphrins such as tetraphenyl porphine and copper chlorophyllin; thionine dyes, such as methylene blue and thionine; amines such as triphenyl amine; polycyclic aromatic and heterocyclic compounds such as anthracene, pyrene, fluorene, acridene, carbazole and their basic derivatives; aromatic hydroquinones, diamino phenyl oxazoles and triazines.

Typical Lewis acids include among others indranthrone dyes such as anthrazol blue IBC, azo dyes such as naphthol blue black B, benzoazurin G; aromatic compounds such as 2,4,7-trinitro fluorenone, tetrachloro phthalic anhydride, chloranil, fiuoranil, anthraquinone and 2 decyanomethylene-l,3-indane dione. The Lewis base nigrosine is a particularly preferred material in obtaining optimum rejuvenation of electrical properties for positively charged selenium or selenium alloy photoconductors.

Additional materials may be added to the developer for particular functions. For example, viscosity controlling additives or additives which contribute to fixing the pigment on the copy paper may be employed.

The proportions of the several constituents in the developer may be varied over a wide range depending individual properties of the constituents and operational considerations of the specific development scheme. A significant factor in determining proportions is the speed of development since with higher speeds lower viscosity developers must be used than at lower speed. One skilled in the art may readily determine the appropriate viscosity for any given development speed.

Generally for development speeds of from about to about inches per second, for example, developer viscosities of from about 300 to about 1800 centipoises measused at C. are preferred to provide ease of operation and desired print quality. The viscosity is in part dependent on the pigment loading of the vehicle. As more pig ment is added the viscosity of the developer increases and development speed is lowered. The balance between pigment loading to obtain maximum image density and development speed to maintain maximum development speed may be readily determined by one skilled in the art.

In general, the several constituents may be present in a developer in amounts according to the following weight percentages:

Vehicle (including principal and secondary vehicle) from about to about 90 wt. percent Colorant pigment or dyeup to about 60 wt. percent.

Dispersant-up to about 20 wt. percent.

Hydrogenated vegetable oil derivativefrom about 0.1

to about 1.0 percent.

Lewis acid or basefrom about .05 to about 1.0 wt. percent.

The hydrogenated vegetable oil derivative should be present at least in an amount necessary to yield the superior and unexpected recycling ability disclosed herein but should not be present in quantities which will provide excessive gelling in the developer since with any significant amount of gelling the developer ceases to function in the desired manner.

Within the broad range of proportions set forth above a preferred range of proportions for the constituents of the developer providing good print quality and ease of operation at development speed of from 2 to about 20 inches per second are the following:

Total vehiclefrom about 65 to about 85 wt. percent. Primary vehiclefrom about 20 to about 85 wt. percent. Secondary vehiclefrom about 0 to about wt. percent. Pigmentfrom about 15 to about 35 wt. percent. Dispersant-from about 5 to about 15 wt. percent. Hydrogenated vegetable oil derivativefrom about 0.25

to about 0.85 wt. percent. Lewis acid or base-from about 0.1 to about 0.50 wt.

percent.

The developers of this invention may be prepared by simply mixing the several constituents. However, to provide homogeneity it is generally preferred to combine the constituents of the vehicle first while heating them and then adding dispersant, pigment and additive.

In addition to providing the superior and unexpected cycling ability to development systems employing reusable or cycling photoconductors additional advantages of the developers of this invention which are common to all liquid development systems are unexpectedly achieved. These superior and unexpected results are attributed in large part to the addition of small quantities of hydrogenated vegetable oil derivatives to the developer.

The addition of small quantities of the additives described herein to the class of developers of this invention provides surprisingly sharper images of improved resolution with less feathering of the developer into the copy sheet. It is also surprisingly observed that pigment suspension in the developer is improved and that an increased dwell time for transfer is obtained for prints of similar quality. With respect to the latter property, the dwell time is that time in which the developed image on the photoconductor and the paper to which the developer is to be transferred are in contact. As this dwell time increases the sharpness of the print decreases. With the developers disclosed herein an increased dwell time for prints of similar quality is obtained. This result reduces the necessity for immediate separation of the photoconductor and transfer paper.

As previously stated the mechanism by which the fatty acid esterified alkylene diols and triols operate to produce the surprisingly superior and unexpected results is not fully known. However, the thixotropic properties do not constitute the lone contribution of the additives of this invention since other thixotropic or gelling agents do not yield results equivalent in character to those obtained with the hydrogenated vegetable oil derivative of this invention. It is, however, known that the thixotropic or gelling property is in part a significant contribution to some of the surprisingly superior results. Thus, the developer is fluid at times when high fluidity is desired such as in the doctoring, applicating, transfer and cleaning steps. During other operations the developer exhibits a rigid gel-like structure and in the recycling photoconductor system builds up to a maximum thickness on the photoconductor and permits transfer of substantially only the developer applied in any given cycle.

An alternative technique of achieving improved cycling ability of a photoconductor in a polar liquid development system is to add the hydrogenated vegetable oil derivatives of this invention directly to the photoconductor after transfer of developer to copy paper. Any suitable dispensing device which will add about 300 to about 500 mg. of the additive per square meter of photoconductor surface may be used. After the addition of the additive in this manner the developer and additive on the surface of the photoconductor are mixed or spread out to uniformly distribute the developer film and additive. This mixing may be provided by the cleaning step which may comprise an absorbent web type cleaning. After reaching a static condition the additive may be added only every two or three cycles. The quantity of hydrogenated vegetable oil should be monitored since excessive quantities added in this fashion may tend to gel the developer film so that added developer on each cycle will tend to solidify and cake. The additive is preferably added immediately after transfer of the developer to copy paper since if it is added before development some of it will be transferred to the applicator roll and subsequently to the development section and since the quantity of additive is not monitored with respect to the developer composition excessive quantities may be present on the applicator surface or in the developer resulting in undesirable caking or gelling. In each cycle, however, it is observed that some of the additive is taken up by the absorbent cleaning web.

All the electrostatographic imaging surfaces that may be employed in a development system which uses a reusable or cycling imaging surface possess the problems mentioned above to varying degrees and the novel developers of this invention are effective in minimizing these problems. Typical of such imaging surfaces are hard surfaced, impervious structure photoconductors, such as selenium or selenium alloy drums or plates, cadmium sulfo selenide glass binder photoconductors such as phthalocyanine enamels. In these recycling systems any suitable cleaning system may be employed. A typical cleaning system lightly scrubs the ink film on the photoconductor surface obliterating the image pattern by smearing the developer over the surface. The residual developer is subsequently picked up by an absorbent web which absorbs the developer. For example, a squeegee roller may be used as the scrubbing or obliterating device and an absorbent web wrapped around a portion of the photocon- 11 ductor drum and moving slowly counter to the direction of rotation of the drum may be used. A corona discharge device may be positioned near the end of the absorbent Web to neutralize any residual charges. The residual static ink film has a thickness of about 1-3 microns.

As previously discussed the developer of this invention may be employed in many configurations of the basic liquid development systems. Included within this group but not limited thereto are systems employing reusable and single use photoconductors, systems where the developed image is transferred to a receiver surface and systems employing a single use photoconductor. A particularly preferred system because of simplicity, ease of operation and one which provides copies on ordinary paper is one employing a single use photoconductor such as zinc oxide or phthalocyanine paper backed binder layers wherein the image is formed and developed on the photoconductor and the liquid developer is transferred to ordinary copy paper in image configuration.

DESCRIPTION OF PREFERRED EMBODIMENTS The following examples further define, describe and compare preferred materials, methods and techniques of the present invention: Examples I, V, VI are included for comparative purposes to show the surprisingly superior and unexpected results obtained in the practice of this invention. All parts and percentages are by weight unless otherwise specified.

Example I A commercial Type E selenium xerographic plate containing a surface layer of selenium about 50 microns thick available from Xerox Corporation, Rochester, NY. is charged and exposed to a light and shadow image in conventional manner. The electrostatic latent image thus formed is developed by moving a patterned surface applicator roll having developing quantities of developer in the depressed portions thereof past the image bearing surface so that liquid developer is pulled out of the depressed portions to the image bearing surface in image configuration. The speed of development is about inches per second. The developer employed is of the following composition:

Parts by weight Drakeol 9 38 Microlith CT Black 38 Rucoflex TG-S 9 Ganex V216 l4 Nigrosine SSJJ 0.3

Drakeol 9 is a mineral oil manufactured by Pennsylvania Refining having a kinematic viscosity of about 15.7l8.1 centistokes at C. and a specific gravity of .85. Microlith CT Black is a resinated predispersed carbon black pigment manufactured by CIBA composed of about 40% carbon black pigment and 60% ester gum resin. Rucofiex TG8 is a triethylene glycol dicaprylate manufactured by Hooker Chemical Company which serves as a solvent for the resinated carbon black pigment and may be regarded as a secondary vehicle in this formulation. Ganex V216 is an alkylated polyvinyl pyrrolidine compound manufactured by GAF Corporation Which serves as a pigment dispersant and may also be regarded as a secondary vehicle. Nigrosine 88]] is a spirit soluble nigrosine dye manufactured by American Cyanamid.

The developer on the photoconductor in image configuration is transferred to copy paper. The selenium plate is then wiped clean with a cotton cloth to remove excess developer. However, all the developer is not removed and after the first cycle a thin film is observed to remain on the plate. The resulation of the first print is about 10 line pairs per millimeter. The procedure outline above is repeated using the same selenium plate. After each cycle the film of residual developer left on the selenium plate is observed to increase in thickness and the print observed on the third cycle has lost all fine detail, the resolution having dropped to only about 2 line pairs per millimeter.

Example II The same procedure as employed in Example I is repeated except that a small quantity of glyceryl tri-( l2-hydroxystearate) is added to the developer used in Example I to provide a developer of the following composition by weight:

Parts by weight Drakeol 9 38 Microlith CT Black 38 Rucoflex TG-8 9 Ganex V216 l4 Nigrosine SSJJ 0.3 Glyceryl tri-(lZ-hydroxystearate) 0.7

The print obtained from the first cycle has a resolution of about 10 line pairs per millimeter. After repeated cycling the film of residual developer is observed to build up to a level of about 3 microns after 4 cycles and remain substantially at the level for further cycling. Resolution is observed to diminish to about 6 to 7 line pairs per millimeter after about 25 cycles and to remain stable at this level for an additional prints.

Example III The procedure of Example I is repeated with the exception that immediately after transfer of developer from the selenium plate to the copy paper in image configuration a small quantity of glyceryl tri-(lZ-hydroxystearate) is applied in powdered form to the photoconductor plate at a rate of about 400 mg./ sq. meter. The photoconductor is wiped with a cotton cloth to provide uniform distribution of the powdered additive and the residual developer. The resolution obtained on the first print is the same as in Example I, 10 lp./mrn. After repeated cycles the film thickness is observed to be about 3 microns thick. The resolution is observed to slowly decrease to about 6 to 7 line pairs per millimeter after 15 cycles and remain at this level for an additional ten cycles.

Example IV The procedure of Example III is followed with the exception that the powdered glyceryl tri-(lZ-hydroxystearate) is applied immediately after the first transfer to copy paper and is thereafter applied to the photoconductor after every other cycle. Results similar to those in Example III are observed.

Example V The procedure of Example I is repeated with the exception that about one part by weight of Bentone 34 is added to the developer employed in Example 1. Ben tone 34 is dimethyl dioctadecyl ammonium bentonite available from National Lead Company. After repeating cycling results similar to those described in Example I are observed.

Example VI A zinc oxide paper backed binder layer photoconductor is charged and exposed in conventional manner. The developer described in Example I, except with the omission of nigrosine, is applied in doctored configuration to a rotatably mounted cylindrical roll having a patterned surface such that developer is present in the valleys or depressed portions of the applicator while the raised portions are substantially free of developer. The applicator roll so loaded with developer is rolled across the zinc oxide paper bearing an electrostatic latent image to develop the image by pulling developer from the applicator onto the zinc oxide paper in image configuration. The developer on the zinc oxide paper is transferred to receiver paper in image configuration by moving the zinc oxide paper and receiver paper in contact along the en- 13 tire length of the image bearing surface. The print obtained on the receiver paper had a resolution of line pairs per millimeter and an image density of 0.8.

Example VII The procedures of Examples I and VI are repeated with the exception that the developer is of the following compositions by weight:

Parts by weight Pale 170 34 Rucofiex TG-8 33 Ganex V216 8 Microlith CT Black 24 Thixcin R 0.6

Pale 170 is an oxidized castor oil and Thixcin R is hydrogenated castor oil which is principally glyceryl tri-( l2- hydroxystearate) both available from Baker Castor Oil Company. Results substantially the same as those in Examples II and VII respectively are observed.

Example IX The procedures of Examples I and VI are repeated with the exception that the developer is of the following compositions by weight:

Parts by weight Oleic acid 66 VM 550 33 Propylene glycol mono(hydroxystearate) 0.5

The procedures of Examples I and VI are repeated with the exception that the developer is of the following compositions by weight:

Parts by weight Sunflower seed oil 38 Rucofiex TG-S 9 Ganex V216 14 Microlith CT Black 38 Ethylene glycol mono(hydroxystearate) 0.8

Results substantially the same as those in Examples II and VII respectively are observed.

Example XI The procedures of Example I and VI are repeated with the exception that the developer is of the following compositions by Weight:

Parts by weight Flexicin-P6 62 Ganex V216 30 Statex B12 8 Diethylene glycol di-(lZ-hydroxystearate) 0.4

Flexicin-P6 is butyl acetyl ricinoleate available from Baker Castor Oil Company. Statex B12 is a carbon black pigment available from Columbian Carbon Company. Results substantially the same as those in Examples II and VII respectively are observed.

14 Example XII The procedure of Example I is repeated except that 0.3 part by weight of 1,6 hexanediol diricinoleate is added to the developer composition. The print obtained from the first cycle has a resolution of about 9 line pairs per millimeter. With repeated cycling a film of developer is observed to build up on the xerographic plate to about 3 microns and remain substantially at that level. The resolution of the 4th through the 25th print is observed to be about 6 line pairs per millimeter.

Example XIII The procedure of Example I is repeated except that 0.4 part by weight of 1,8 octanediol monolauratc is added to the liquid developer composition. Results substantially the same as those in Example XII are observed.

Example XIV The procedure of Example I is repeated with the exception that the developer is of the following compositions by weight:

Parts by Weight Sunflower seed oil 38 Rucofiex TG8 9 Ganex V216 14 Microlith CT Black 38 1,8-octanediol monolinoleate 0.3 1,5-pentanediol dioleate 0.5

The print obtained from the first cycle has a resolution of about 9 line pairs per millimeter. After 3 cycles the film of residual developer is observed to build up to about 3 microns and remain substantially at that level for an additional 10 cycles. The resolution is observed to diminish to about 6 line pairs per millimeter after the third cycle and remain stable at this level for an additional 10 prints.

Example XV The procedure of Example XIV is repeated except that 0.8 part by weight of a mixture of equal parts by weight of 1,2,4-butanetriol 2 monooleate and 1,10-decanediol dilinoleate is substituted for the mixture of 1,8-0ctanediol monolinoleate; 1.5 pentanediol dioleate. Results substantially the same as those in Example XIV are observed.

The above comparative examples show the surprisingly superior and unexpected results, particularly the print quality, obtained in the practice of the invention herein described. Comparison of Examples I and 11 clearly show the distinct advantages in a recycling photoconductor development system. Examples III and IV show the surprising improved cycling ability of a photoconductor due to the simple addition of the additive directly to the photoconductor. Example V shows that conventional gelling or thixotropic agents do not yield results similar to those obtained in the instant invention and is offered to support the conclusion of surprising and unexpected results. Examples VI and VII show the surprising improvements obtained in the practice of this invention in a polar liquid development system employing a single use photoconductor.

Although specific materials and operational techniques are set forth in the above exemplary embodiments using the developer composition and development techniques of this invention these are merely intended as illustrations of the present invention. There are other developer materials and techniques such as those listed above which may be substituted for those in the examples with similar results.

Other modifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure which modifications are intended to be included within the scope of this invention.

What is claimed is:

1. The method of cyclically developing electrostatic latent images on a reusable electrostatographic imaging surface comprising the steps of forming an electrostatic latent image on the imaging surface, developing the image with a liquid developer having a conductivity of from about 10- to about 10- (ohm-cm.)- said developer comprising a liquid vehicle carrying a colorant member selected from the group consisting of a pigment and a dye, and having dispersed therein a developer build-up inhibiting proportion of a material selected from the group consisting of nonliquid fatty acid esterified alkylene diols, triols and mixtures thereof, transferring the developer from the imaging surface to a receiving surface in image configuration, distributing residual developer over the imaging surface, uniformly wiping the imaging surface to prepare it for the next imaging cycle and repeating the steps of forming an electrostatic latent image, developing the image, transferring developer to a receiving surface, distributing residual developer and wiping the imaging surface at least one additional time.

2. The method of claim 1 wherein said nonliquid material has the general structural formula:

where R is selected from the group consisting of H and CH (CH z is from to 7, R is selected from the group consisting of (CH2); and (CHzhHC- 2: is from 0 to 8, y is from 0 to 7; R R and R are selected from the group consisting of H and and R is the carbon chain of a fatty acid of from about 12 to about 22 carbon atoms, provided at least 1 of R R and R includes the group 3. The method of claim 1 wherein said nonliquid material is a lower alkyl ester of hydrogenated hydroxyoctadecenoic acid.

4. The method of claim 1 wherein said nonliquid material is ethylene glycol mono(hydroxy stearate) 5. The method of claim 1 wherein said nonliquid material is propylene glycol mono(hydroxy stearate).

6. The method of claim 1 wherein said nonliquid material is glyceryl tri-(12-hydroxystearate).

7. The method of claim 1 wherein said nonliquid material is hydrogenated castor oil.

8. The method of claim 1 wherein said electrostatographic imaging surface is a photoconductor.

9. The method of claim 8 wherein said photoconductor is selected from the group consisting of selenium and selenium alloys.

10. The method of claim 1 wherein said liquid developer further comprises a member selected from the group consisting of Lewis acids and Lewis bases.

11. The method of claim 1 wherein said nonliquid material is present in an amount of from about 0.1 to about 1 percent by weight of the liquid developer.

12. The method of cyclically developing electrostatic latent images on a reusable electrostatographic imaging surface comprising the steps of forming an electrostatic latent image on the imaging surface, developing the image with a liquid developer having a conductivity of from about 10* to about 10- (ohm-cm.)- said developer comprising a liquid vehicle carrying a colorant member selected from the group consisting of a pigment and a dye, transferring the developer from the imaging surface to a receiving surface in image configuration, providing from about 300 to about 500 milligrams per square meter of imaging surface of a material selected from the group consisting of nonliquid fatty acid esterified alkylene diols, triols and mixtures thereof, substantially uniformly distributing the residual developer and said nonliquid material over said imaging surface and wiping said imaging surface to prepare it for the next cycle.

13. The method of claim 12 wherein said nonliquid material has the general structural formula:

where R is selected from the group consisting of H and CH (CH z is from 0 to 7, R is selected from the group consisting of x is from O to 8, y is from 0 to 7; R R and R are selected from the group consisting of H and and R is the carbon chain of a fatty acid of from about 12 to about 22 carbon atoms, provided at least 1 of R R and R includes the group 14. The method of claim 12 wherein said nonliquid material is a lower alkyl ester of hydrogenated hydroxyoctadecenoic acid.

15. The method of claim 12 wherein said nonliquid material is ethylene glycol mono(hydroxy stearate).

16. The method of claim 12 wherein said nonliquid material is propylene glycol mono(hydroxy stearate).

17. The method of claim 12 wherein said nonliquid material is glyceryl tri-(lZ-hydroxystearate).

18. The method of claim 12 wherein said nonliquid material is hydrogenated castor oil.

19. The method of claim 12 wherein said nonliquid material is present in an amount of from about 0.1 to about 1 percent by weight of the liquid developer.

References Cited UNITED STATES PATENTS 3,472,676 10/1969 Cassiers et al. 117-37 L X 3,438,904 4/1969 Wagner 252-62.1 3,417,019 12/1968 Beyer 252-621 3,392,707 7/1968 Mark 117-37 L X 3,245,381 4/ 1966 Brenneisen et al. 117-37 L X 3,241,957 3/1966 Fauser et al. 252-621 3,084,043 4/1966 Gundlach 117-37 L X 3,079,270 2/ 1963 Cortez 252-62.1

GEORGE F. LESMES, Primary Examiner J. P. BRAMMER, Assistant Examiner US. Cl. X.R.

96-1 -L Y, 1.4; 117-37 L E; 252-621 

